HG: Detect branch state changes

Branch creation is detected by CRUC-3809, but we also need to detect/track:

• the head commit of a branch (latest head is fine)
• going inactive
• being closed
• being re-opened

This should occur at the end of a batch in the repo life-cycle, can use hg branches -v to list branches and get the head commit and state.

Allow for different processing to be plugged in to these states (KISS), mostly will be CRUC-3408